1. Field of the Invention
The present invention is a method for forming a model simulating sedimentary filling of basins over large time and space scales.
The method according to the invention more particularly relates to the formation of a numerical stratigraphic model allowing two or three-dimensional (2D or 3D) simulation of the multilithologic filling of a basin in order to simulate the stratigraphic response of the sedimentary system to variations in time of the eustasy, the subsidence, the sediment supply and the physical parameters governing sediment transportation in the basin.
2. Description of the Prior Art
The advances recently made in geology, which gave birth during the last twenty years to seismic stratigraphy, then to genetic stratigraphy, have deeply modified the understanding of the history of sedimentary filling of basins over substantial time and space by showing the crucial influence of three main parameters: eustasy, tectonics and sedimentary flow.
Many models and notably deterministic numerical models have been formed in order to apprehend the geometric and lithologic implications of these new approaches.
These numerical models simulate the transportation and the sedimentation (or the erosion) of sediments in the basin and are based on a description of the nature, from an estimation of the eustasy, the subsidence and the sediment supply. Eustasy is the variation of the surface of the oceans simultaneously observed on all of the earth's surface, and subsidence is the absolute displacement in time of the bottom of a sedimentary basin in relation to a fixed reference level. These models thus allow testing the influence of various concepts (relationship between climate and eustasy, etc.) on the layout of the sedimentary units. Furthermore, applied to real cases, these models allow testing the coherence of the parameters introduced in the model, such as the eustatic and tectonic variations, and to reinforce the geologic interpretation of the studied basin.
A model is expected to define the main facies variation tendencies (variation of the sand/clay ratio, carbonate content, etc.) within the genetic units. It is therefore necessary to design a multilithologic modelling. The model must be able to simulate the transportation and the sedimentation of various siliciclastic (sand, clay, etc.) and carbonate (reef, carbonate deep-sea ooze, bioclasts, etc.) lithologies so that these facies variation tendencies are a result of the model, independent of geologic a priorisms such as sandy alluvial plain and clayey marine domain. In this ideal model, the siliciclastic sediments are carried along by rivers and ocean currents into the basin, whereas the carbonates are produced in marine domains, taking into account the bathymetry, the turbidity of the water and the action of the waves. After being carried along to the basin boundaries or produced in the basin, the sediments are transported, then sedimented.
There are three main deterministic model families which govern sediment transportation:    the particulate models based on the solution of the movement of particles (calculation of the flow of water, then relation between water flow and sediment flow),    the diffusive models based on a diffusion equation where the definition of the diffusion coefficient is refined (taking account of several lithologies, of the water flow, of the deposit environment, etc.), and    the geometric models based on a definition of the geometric profile of the deposit environments (length and slope of the alluvial plain, etc.) or on a geometric definition of the sedimentation rate (exponential decrease in marine domains, etc.).
The particulate models use a careful description of the sedimentary processes and are therefore as chaotic as nature. They essentially allow obtaining simulations on reservoir scale (length of the order of 0.5 to 50 km and duration of the order of 5 to 500 ka). The diffusive and geometric models both provide rough approximations of natural processes. They are much more stable than the reality they account for, but they provide only a smoothed estimation of nature. These models are preferably applicable on basin scale (length of the order of 10 to 1000 km and duration of the order of 0.1 to 100 Ma). The geometric models, based on an approximation of the basin setting, are essentially applicable in simple 2D cases wherein the subsidence, the nature of the sediments and the climate do not disturb the definition of the equilibrium profile too much. More general, the diffusive models, based on an approximation of the physics of the sediments, are applicable in 3D and can deal with transportation and sedimentation of multiple lithologies.
Various known diffusive models are for example described by:    Kenyon and Turcotte, 1985, Morphology of a Delta Prograding by Bulk Sediment Transport. In Geol. Soc. Amer. Bull., 96, 1457–1465,    Begin, Z. B., 1988, Application of a Diffusion-Erosion Model to Alluvial Channels which Degrad Due to Base-Level Lowering. Earth Surface Processes and Landforms, 13, 487–500,    Rivenaes, J. C., 1988, Application of a Dual-Lithology Depth-Dependent Diffusion Equation in Stratigraphic Simulation. Basin Research, 4, 133–146.
One advantage of diffusive models is that they allow returning to geologic concepts by quantifying certain relations such as the variable duration of the progradation and retrogradation stages of the genetic units, or the evolution of the sandiness as a function of either the bathymetry, or the prograding or retrograding tendency of the longshore.
The goal of the prior models is essentially a numerical approximation of theoretical concepts along profiles in 2D.
French patent 2,744,224 filed by the assignee describes a method for simulating the filling of a sedimentary basin. From known data on the architecture of a basin and from measured data: well logs, seismic surveys, etc., a set of input data is formed, which relates to the accommodation created by subsidence and eustasy, to the supply and production of fluvial or marine sediments, and to physical transportation parameters such as diffusion coefficients of the various lithologies. This data set is applied to a numerical model. The results, that can be deduced on the geometry and the lithologies of the sedimentary units, are compared with the measured data and the input data are refined step by step by inversion.